There are various types of control systems for three-phase rotary machines. These control systems include an inverter comprised of three pairs of high- and low-side switching elements; each pair of high- and low-side switching elements is provided for a corresponding phase of a three-phase rotary machine. The control systems perform on-off operations of the high- and low-side switching elements of each pair to convert a DC voltage supplied from a DC power source to an AC voltage, and supply the AC voltage to the three-phase rotary machine as an output voltage of the inverter, thus outputting torque that rotates a rotor of the three-phase rotary machine.
In the control systems, there is known a control system for a three-phase motor as an example of these three-phase rotary machines, which is disclosed in Japanese Patent Publication No. 4983322.
The control system disclosed in the Patent Publication uses a negative bus connected between a negative DC input terminal of the inverter and a negative terminal of the DC power source as a feedback loop to measure a peak value of a current flowing through the negative bus; this current will be referred to as a bus current.
For example, the control system includes a current measuring circuit comprised of a shunt resistor and a peak-hold circuit. The shunt resistor is provided in the negative bus. The peak hold circuit is operative to measure a voltage drop across the shunt resistor while the bus current is flowing through the shunt resistor, measure the bus current as a phase current using the voltage drop, and hold a peak value of the phase current. That is, a current for one of the three-phases of the three-phase rotary machine will be referred to as a phase current.
Based on the peak value of a phase current, the control system corrects a command value for the output voltage of a three-phase inverter as a feedforward manipulated variable therefor; the command value is proportional to the rotational speed of the three-phase motor. The control system generates, based on the corrected command value, drive signals each having a controllable duty cycle, i.e., a controllable on-pulse width for each cycle using PWM (Pulse Width Modulation). Then, the control system individually drives each of the high- and low-side switching elements according to a corresponding one of the drive signals to control an output voltage of the three-phase inverter to be supplied to the three-phase motor.